Although this massive creature is classified as a whale, it is not the ancestor of modern whales. When it was first unearthed in the 1830s, paleontologists thought it was some kind of large prehistoric reptile.

Basilosaurus averaged about 15 meters (50 feet) long, but some individuals may have attained lengths of up to 24 meters (80 feet). The body of Basilosaurus did not resemble those of modern whales very closely. Its backbone had very elongated vertebrae, and its head was relatively small. The narrow skull contained 44 teeth of various shapes: pointed, conical teeth in front, and saw-edged teeth in the back. This configuration made it possible for Basilosaurus to catch and eat fish, hard-shelled mollusks, and crustaceans.

Although its cylindrical body could flex from side to side, Basilosaurus swam mainly by undulating up and down. Its forelimbs were flippers, but had an elbow joint that could pivot like a seal's. Basilosaurus was able to pursue its prey into holes and crevices underwater.